1. Field of the Invention
The present invention relates to imaging systems. More specifically, the present invention relates to radar imaging systems.
2. Description of the Related Art
Imaging techniques are well known and widely used in the art. Certain imaging technologies are better suited for particular applications. For example, radar imagery is widely used for surveillance and reconnaissance as well as target tracking and identification. For radar and other imaging technologies, the ability to clearly resolve and discriminate targets may be essential in meeting objectives specified for a particular application.
One such application involves xe2x80x98real beam ground mapping.xe2x80x99 Real beam ground mapping involves scanning an area, e.g., the earth""s surface, using a scanning antenna or an electronically scanned antenna. Returns from an illumination of the surface are then examined for xe2x80x98back-scatterxe2x80x99 or reflections therefrom. As the beam is scanned in azimuth, information is collected with respect to the range direction. At each beam position, the distance of various scatterers may be ascertained for each range cell. This information may then be displayed in a real beam ground mapped image.
While range data may be resolved with adequate resolution, currently, resolution of azimuth data with comparable resolution has proved to be problematic. This is due to the fact that azimuth resolution is limited to the width of the antenna beam and the corresponding cross range resolution degrades as a function of range. Accordingly, the poor resolution of conventional real beam mapping systems limits the ability of the system to discriminate scatterers.
Conventional doppler beam sharpening (DBS) or Synthetic Aperture Radar techniques may be used to improve the azimuth resolution, but these require excessive frame times if the coverage includes regions close to the velocity vector.
xe2x80x9cSuper resolutionxe2x80x9d techniques are widely used to sharpen the radar imagery. However, the quality achieved is scene dependent and is not robust.
Hence, a need remains in the art for an improved system or method for providing ground mapped images, in a timely manner, that include regions near the velocity vector. Specifically, a need remains in the art for a system or method for providing enhanced cross-range (azimuthal) resolution with a frame time similar to that of a real beam ground mapping radar system.
The need in the art is addressed by the imaging system and method of the present invention. The inventive system is adapted for use with an electronically scanned (e.g., synthetic aperture array radar) antenna. The inventive method includes the steps of activating the antenna to generate a beam of electromagnetic energy; causing the beam to scan over a predetermined scan volume consisting of a predetermined range of scan angles relative to a reference vector; and generating multiple simultaneous beams of electromagnetic energy over a subset of the predetermined range of scan angles.
In a specific illustrative embodiment, the inventive method further includes the steps of determining azimuth beam positions for the scan volume; computing a dwell time for beam sharpening for each of the azimuth beam positions; computing a maximum pulse repetition interval for each of the beam positions; computing a minimum pulse repetition interval common to all of the beam positions; selecting a pulse repetition interval for each beam position as the greatest multiple of the minimum pulse repetition interval which is less than the maximum pulse repetition interval; interleaving azimuth beam positions within the scan volume which allow for a synchronization of pulses; and generating the multiple beams at the interleaved beam positions in accordance with the dwell times.
Thus the present invention provides an intra-pulse repetition interval (PRI) agile beam technique for enhanced resolution, which can be used at aspect angles near the velocity vector of a host vehicle. At these scan angles, the bandwidth of the clutter is narrower than at higher scan angles and allows large PRIs without degradation from Doppler ambiguities. In accordance with the present teachings, sequential illumination is performed within a PRI to multiple beam locations using an agile beam. The interleaving of beams reduces map formation times compared to conventional techniques using sequential arrays.